In this type of machine one of the difficulties to be overcome when pumping with compression a gas which can condense or crystallize, in clogging of ducts, especially in the discharge region.
During compression, a gas which can condense or crystallize may be subjected to conditions in which it can pass through a change of state, with development of droplets or solid particles (crystals). This is especially frequent in the case of machines acting as a compressor or a vacuum pump and equipped with external or internal refrigeration.
The main problem with this phenomenon is the progressive blocking of ducts which leads to a loss of flow and thus to a damaging increase in internal losses (energy dissipated frictionally and excessive heating of the fluid and the machine). The risk of seizing up is one of the consequences. In some cases this may involve complete blockage of the ducts.
To avoid this phenomenon it is known to inject a certain amount of a dry gas, such as air, nitrogen, etc., into the discharge side of the machine, which limits the partial pressure of the pumped gas and thus avoids the change of state of the components which can condense or crystallize. Even if it is not enough to prevent the change of state, it allows particles to be put into suspension, so avoiding their deposition on the walls, through increasing the discharge flow and thus the flow velocity.
However this does not ensure effective protection for all the walls because the fluid only diffuses progressively in the duct and does not effectively protect either the discharge outlet of the machine or the first part of the downstream duct, which is however the most vulnerable and should be given the best protection.